A self-assembled supramolecular complex is reported to catalyze alkyl-alkyl reductive elimination from high-valent transition metal complexes [such as gold(III) and platinum(IV)], the central bond-forming elementary step in many catalytic processes. The catalytic microenvironment of the supramolecular assembly acts as a functional enzyme mimic, applying the concepts of enzymatic catalysis to a reactivity manifold not represented in biology. Kinetic experiments delineate a Michaelis-Menten-type mechanism, with measured rate accelerations (k(cat)/k(uncat)) up to 1.9 × 10(7) (here k(cat) and k(uncat) are the Michaelis-Menten enzymatic rate constant and observed uncatalyzed rate constant, respectively). This modality has further been incorporated into a dual catalytic cross-coupling reaction, which requires both the supramolecular microenvironment catalyst and the transition metal catalyst operating in concert to achieve efficient turnover.

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C–C and C–O bond cleavage, carbon–carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

The field of Surface Organometallic Chemistry (SOMC) aims to blend the positive attributes of homogeneous and heterogeneous catalysis. Significant insight into heterogeneous systems has been gained over the years through the synthesis, characterization, and application of well-defined surface organometallic catalysts, predominantly supported on silica and alumina. Considerable research efforts have focused on the application of homogeneous methods to the synthesis and characterization of these systems. Homogeneous catalysis has thrived on its ability to electronically and sterically tune ligands to yield desired reactivity and selectivity. Efforts in SOMC, however, have only recently turned to harnessing the stereoelectronic diversity of potential inorganic support materials beyond silica and alumina in order to exert similar control on the reactivity of the organometallic active site. The support material is intrinsically linked to electronic structure and reactivity of heterogeneous organometallic systems in the same way that ligands exert control over homogeneous catalyst systems. The ability to tune the reactivity of heterogeneous catalysts by changing the support is of great value, and it is anticipated that this will represent an area of significant growth in the field. In this Perspective, the use and future of nontraditional catalyst supports, such as sulfated metal oxides, modified silicas, and redox active supports are discussed.

Supramolecular assembly 1 catalyzes a bimolecular aza-Prins cyclization featuring an unexpected transannular 1,5-hydride transfer. This reaction pathway, which is promoted by constrictive binding within the supramolecular cavity of 1, is kinetically disfavored in the absence of 1, as evidenced by the orthogonal reactivity observed in bulk solution. Mechanistic investigation through kinetic analysis and isotopic labeling studies indicates that the rate-limiting step of the transformation is the encapsulation of a transient iminium ion and supports the proposed 1,5-hydride transfer mechanism. This represents a rare example of such an extreme divergence of product selectivity observed within a catalytic metal-ligand supramolecular enzyme mimic.

ABSTRACT:The scope and mechanism of the microenvironmentcatalyzed C(sp 3 )−C(sp 3 ) reductive elimination from transition metal complexes [Au(III), Pt(IV)] is explored. Experiments detailing the effect of structural perturbation of neutral and anionic spectator ligands, reactive alkyl ligands, solvent, and catalyst structure are disclosed. Indirect evidence for a coordinatively unsaturated encapsulated cationic intermediate is garnered via observation of several inactive donorarrested inclusion complexes, including a crystallographically characterized encapsulated Au(III) cation. Finally, based on stoichiometric experiments under catalytically relevant conditions, a detailed mechanism is outlined for the dual supramolecular and platinumcatalyzed C−C coupling between methyl iodide and tetramethyltin. Determination of major platinum species present under catalytic conditions and subsequent investigation of their chemistry reveals an unexpected interplay between cis−trans isomerism and the supramolecular catalyst in a Pt(II)/Pt(IV) cycle, as well as several off-cycle reactions.

This study offers a detailed mechanistic investigation of host-guest encapsulation behavior in a new enzyme-mimetic metal-ligand host and provides the first observation of a conformational selection mechanism (as opposed to induced fit) in a supramolecular system. The GaL host described features a C-symmetric ligand motif with meta-substituted phenyl spacers, which enables the host to initially self-assemble into an S-symmetric structure and then subsequently isomerize to a T-symmetric tetrahedron for better accommodation of a sufficiently large guest. Selective inversion recovery H NMR studies provide structural insights into the self-exchange behaviors of the host and the guest individually in this dynamic system. Kinetic analysis of the encapsulation-isomerization event revealed that increasing the concentration of guest inhibits the rate of host-guest relaxation, a key distinguishing feature of conformational selection. A comprehensive study of this simple enzyme mimic provides insight into analogous behavior in biophysics and enzymology and aims to inform the design of efficient self-assembled microenvironment catalysts.

The chemical and electronic interactions of organometallic species with metal oxide support materials are of fundamental importance for the development of new classes of catalytic materials. Chemisorption of Cp*(PMe)IrMe on sulfated alumina (SA) and sulfated zirconia (SZ) led to an unexpected redox mechanism for deuteration of the ancillary Cp* ligand. Evidence for this oxidative mechanism was provided by studying the analogous homogeneous reactivity of the organometallic precursors toward trityl cation ([PhC]), a Lewis acid known to effect formal hydride abstraction by one-electron oxidation followed by hydrogen abstraction. Organometallic deuterium incorporation was found to be correlated with surface sulfate concentration as well as the extent of dehydration under thermal activation conditions of SA and SZ supports. Surface sulfate concentration dependence, in conjunction with a computational study of surface electron affinity, indicates an electron-deficient pyrosulfate species as the redox-active moiety. These results provide further evidence for the ability of sulfated metal oxides to participate in redox chemistry not only toward organometallic complexes but also in the larger context of their application as catalysts for the transformation of light alkanes.

Understanding the mechanisms of action for base metal catalysis of transformations typically associated with precious metals is essential for the design of technologies for a sustainable energy economy. Isolated transitionmetal and post-transition-metal catalysts on oxides such as silica are generally proposed to effect hydrogenation and dehydrogenation by a mechanism featuring either σ-bond metathesis or heterolytic bond cleavage as the key bond activation step. In this work, an organovanadium(III) complex on silica, which is a precatalyst for both olefin hydrogenation and alkane dehydrogenation, is interrogated by a series of reaction kinetics and isotopic labeling studies in order to shed light on the operant mechanism for hydrogenation. The kinetic dependencies of the reaction components are potentially consistent with both the σ-bond metathesis and the heterolytic bond activation mechanisms; however, a key deuterium incorporation experiment definitively excludes the simple σ-bond metathesis mechanism. Alternatively, a twoelectron redox cycle, rarely invoked for homologous catalyst systems, is also consistent with experimental observations. Evidence supporting the formation of a persistent vanadium(III) hydride upon hydrogen treatment of the as-prepared material is also presented.